There are a wide variety of metal oxides and such material oxides are used for various applications. Indium oxide is a well-known material and is used as a material for transparent electrodes which are needed for liquid crystal display devices or the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such a metal oxide is already known (see, for example, Patent Documents 1 to 4, Non-Patent Document 1, and the like).
Not only single-component oxides but also multi-component oxides are known as metal oxides. For example, InGaO3(ZnO)m (m: natural number) having a homologous phase is known as a multi-component oxide semiconductor including In, Ga, and Zn (see, for example, Non-Patent Documents 2 to 4 and the like).
Furthermore, it is confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide can also be applied to a channel formation region of a thin film transistor (see, for example, Patent Document 5, Non-Patent Documents 5 and 6, and the like).